This invention relates to the manufacture of halophosphate phosphors such as are used in fluorescent lamps.
Typically, halophosphate phosphors have been prepared by firing a raw mix containing calcium hydrogen orthophosphate (which has been precipitated as a di-hydrate and then converted to a de-hydrate), calcium carbonate, calcium fluoride, ammonium chloride, and suitable compounds of activator metals. The activators are generally antimony and manganese. It is well known to substitute other alkaline earth metals for some or all of the calcium. It is also well known that, in the preparation of such phosphors, the crystal properties of the phosphor are highly dependent upon the crystal properties of the calcium hydrogen orthophosphate and that good crystals of the proper range of sizes are essential to good performance of the phosphor in a lamp.
Generally, calcium hydrogen orthophosphate is precipitated in a di-hydrate form (CaHPO.sub.4. 2H.sub.2 O) from a mixture of calcium chloride and diammonium phosphate solutions (calcium nitrate has occasionally been used in place of the calcium chloride). The precipitated crystals are generally collected and separated from the mother liquor (the mother liquor consists primarily of ammonium chloride solution together with any unreacted calcium chloride or diammonium phosphate) by filtration. The crystals may be washed at this time to provide further removal of the mother liquor. The di-hydrate crystals are then converted to de-hydrate (anhydrous calcium hydrogen orthophosphate) crystals by gently agitating the crystals in a warm (80.degree.-100.degree.C), slightly acidic solution (pH initially about 4.5). The crystals are then collected (typically by filtration), washed and dried and used as part of the raw mix which is fired to form the halophosphate phosphor.
The effects of precipitation conditions on the particle size of the final phosphor have been recognized in, for example, U.S. Pat. No. 3,242,369 issued to king, et al., on Mar. 22, 1960. Heretofore, however, it had apparently been assumed that filtering conditions did not have a significant effect on particle size and the main emphasis had been on gentle handling to avoid mechanical breaking of the crystals.